Two proteins, barnase, the extracellular ribonuclease of Bacillus amyloliquefaciens, and barstar, its intracellular inhibitor, are used as a model system for the study of protein folding and protein-protein interactions. Barnase is one of an homologous group of ribonuclease occurring in both prokaryotes and eukaryotes. Recombinant DNA techniques are being applied to the project with three major aims: (1) to facilitate production; (2) to examine the structural and control sequences of the genes; and (3) to tailor specifically designed modifications in the sequences to test theories of protein folding. The lethal effect of the cloned wild-type barnase gene in either E. coli or B. subtilis can be repressed by expression of the barstar gene placed on the same plasmid. E. coli plasmid vectors have been devised for both proteins and both can now be obtained essentially pure in 100 mg quantities. DNA and amino acid sequences are known for both and the x-ray structure of barnase has been refined. Preliminary studies suggest that the structure of barstar may be solvable by NMR. At least one Inactive barnase mutant forms a normal complex with barstar. A simple procedure for oligonucleotide-directed mutation using only plasmid DNA has been devised and applied to both genes.